How can I add another layer to my Surfer legend?

In the Layers in Legend section, click the Edit List button in the Edit layer list field.

In the Layers for Legend dialog, choose the layer you wish to add to the legend from the Available layers to Selected list by using the button.

Click OK to close the dialog and add that layer to the legend.

Now that you have multiple layers in the legend, you can display a title for each section displaying which layer the legend pertains to (in the General section check the box next to Show layer name). You can edit the layer properties for each section by selecting the layer from the Layers to edit field in the Layers in Legend section and then editing the properties for that layer in the section below.